A captive market visual novel is basically a visual novel designed for a particular market that is kind of 'captured' in the sense that it has its own specific characteristics and preferences. Visual novels are interactive fiction games that use graphics, text, and sometimes audio to tell a story. In the case of a captive market one, it could be for a market that is into very specific types of stories, like ones set in a post - apocalyptic world with a very unique take on the genre, or a market interested in a particular kind of character development that is not common in general visual novels.
Well, a captive market visual novel is a visual novel which has a defined and rather exclusive market. It might be related to a certain genre or theme that has a dedicated but limited audience. It's like a story - based game with visuals that focuses on appealing to a group that might not be as mainstream as others. Maybe it's based on a very specific historical period or a particular type of fictional world that only a select few are really into.